Conventionally, a magnet-embedded motor (hereinafter, IPM motor) using reluctance torque other than magnetic torque has been used as a high efficiency motor. The reluctance torque is a force generated using saliency of d-axis inductance (Ld) and q-axis inductance (Lq), and permanent magnets have been often arranged in a V-shape so that Ld and Lq have the saliency. However, since arranging the permanent magnets in a V-shape is not sufficient to ensure a uniform magnetic flux flowing between a magnet and a stator, the consequent increase in cogging torque has led to increase in torque ripples and vibrations during motor operation.
Thus, to reduce the cogging torque, a shape of rotor may be changed as shown in Japanese Patent Application Laid-Open No. 2013-99193, shapes of teeth of a stator may be changed as shown in Japanese Patent Application Laid-Open No. 2011-234601, or a slit may be formed in the rotor as shown in Japanese Patent Application Laid-Open No. 2011-101595.
There have been many cases in which cogging torque has been reduced by optimizing a distance between ordinary magnetic poles, a distance between magnets forming each magnetic pole, angles of the magnets, and lengths of the magnets in the arrangement of magnets embedded in the rotor. In this case, the most efficient way is to change a distance between the magnetic poles. This is because the cogging torque is an attractive force generated between the magnet and the stator, and thus it is necessary that the magnetic flux flowing in the teeth be uniform as much as possible.
For example, when the distance between the magnetic poles is too long, the magnetic flux flowing in the teeth is not uniform, and thus the cogging torque is increased. On the other hand, when the distance between the magnetic poles is too short, the cogging torque is reduced, but a short circuit of the magnetic flux between the magnets is generated, an induced voltage is reduced, and thus a motor efficiency is degraded. Therefore, when determining the distance between the magnetic poles, many analyses or evaluations have been necessary, and due to the changed magnetic flux, the induced voltage is reduced, resulting in many cases in which the efficiency of the motor is degraded.